1. Field of the Invention
The present invention relates to drill bits and methods for reducing formation fluid invasion in permeable formations and for improved drilling in plastic formations and more particularly to a new bit and method in which formation cuttings are received into a cavity inside the bit and then circulated to the top of the borehole.
2. Description of the Related Art
In rotary drilling of earth formations, it has long been the practice to irrigate the cutting face of the drill bit with drilling fluid during drilling. In the usual case, drilling fluid is injected into a drill string at the top of the borehole. A drill bit is suspended from the lower end of the drill string. The bit includes a plurality of openings, sometimes formed as nozzles, on the cutting face thereof to communicate drilling fluid to the space between the drill bit and the bottom of the borehole being cut. The fluid then flows up the annulus between the drill string and the borehole carrying chips cut from the borehole bottom to the surface of the borehole. In addition to flushing cut chips from the borehole, the fluid cools the drill bit.
The drilling fluid typically includes a combination of solids, polymers, viscosifiers and other agents to form filtercakes on well bore surfaces. In permeable formations, the filtercake prevents liquid in the drilling fluid from invading the formation. Such liquid is referred to as filtrate. Particles and polymers contained in the drilling fluid are driven into the pores of the formation being drilled to bridge and plug flow paths thereby preventing filtrate from permeating very far into the formation.
For a formation with a given permeability, the extent to which filtrate invasion occurs is a function of: (a) total time the borehole surface is subjected to drilling fluids; (b) the degree to which the formation can be made impermeable to filtrate at the well bore surface; and (c) the flow rate of the drilling fluid circulated in the well bore.
When drilling with conventional bits having polycrystalline diamond cutters mounted thereon, a filtercake forms in the well bore above the lower end of the bore where cutting action occurs. Although filtercake begins forming immediately on a freshly cut surface, the usual drill bit includes cutters positioned so that a filtercake formed on a cut surface made by a leading cutter is at least partially cut into by a closely following cutter. Such action is disadvantageous for two reasons.
First, continuous cutting into the filtercake disturbs the barrier to filtrate presented by the filtercake thereby permitting additional filtrate migration into the formation.
Secondly, the pressure gradient across the filtercake is high, having the well bore drilling fluid pressure on one side and the naturally-occurring formation pore pressure on the other. Under some conditons, this pressure differential effectively strengthens the formation and thus makes cutting into the invaded portion of the formation more difficult than if the cut extended into the formation beyond the formation invasion depth. The lower drilling rate thus exposes the formation to the drilling fluid for a longer period of time thereby causing increased drilling fluid invasion into the formation.
It clearly would be desireable to provide a method and bit for drilling, especially in a permeable formation from which production is contemplated, which minimizes filtrate invasion into the formation while still using drilling fluid, which is necessary to flush cuttings from the borehole and cool the bit.
The above described conditions and associated problems are encountered in permeable formations. Conditions are different, and cause different associated problems, when drilling plastic formations. In plastic or sticky formations, low permeability can prevent substantially all filtrate invasion from the borehole into the formation. When a bit having polycrystalline cutters mounted thereon drills through such a formation, the rock in the formation extrudes around the cutter structure thus balling and clogging the bit and substantially lowering the drilling rate.
It would also be desireable to provide a bit and drilling method which addresses the disadvantages associated with drilling in a plastic formation.
In all types of formations, drilling fluid flow is limited by the space between the surface of the bit and the borehole in which the bit is drilling. Most bits have junk slots which are vertical grooves formed about the circumference of the bit to increase the cross-sectional area through which drilling fluid and rock chips carried therein can flow. It would be desirable to increase the flow rate of drilling fluid thereby increasing the rate at which the bit is cooled and the rate at which chips are flushed from the borehole while minimizing exposure of freshly cut formation to drilling fluid.